The world boss, Thallasion, can be found in the Drowned Elemental Worlds in the Open Seas biome (on the map, it doesn't have any marker/symbol on it) or the other Open Seas biome (colored dark blue on the map and also doesn't have a marker/symbol).

The amount of essences dropped depends on the Uber world the player kills Thallasion in. The higher the Uber, the more fragments drop.